A Scheduling Unit (SU) is a radio resource control (RRC) message carrying a number of System Information Blocks (SIBs) that have the same scheduling requirements (i.e. periodicity). Each SIB contains a set of related system information parameters. Several SIBs have been defined including the Master Information Block (MIB), that includes a limited number of most frequently transmitted parameters, and the Scheduling Block (SB) that mainly indicates when the other system information is transmitted i.e. start times. The MIB is carried on a Broadcast Chanel (BCH) while all other SIBs (including the SB) are carried on a Down Link Shared Chanel (DL-SCH).
The system information (SI) is transmitted periodically by the Base Station (BS) or the evolved Node B (eNB) and must be read by UEs both in IDLE mode and Connected mode.
UEs which have been powered on and are therefore in one of these modes are informed about a coming SI change as described above, either by paging or by BCCH-RNTI. Most of these UEs have stored the previous SI and can continue with this SI until the modification period boundary. The Broadcast Control Chanel (BCCH) modification period is expected to be in the order of several seconds, maybe up to some tens of seconds. This is to allow reliable reading of SI which includes the possibility to receive retransmissions of the SUs.
While the length of the BCCH modification period is considered unproblematic, initial reading of SI can lead to very long delays, for the reason described in the following.